The inositol 5-phosphatase SHIP is expressed as 145 and 135 kDa proteins in blood and bone marrow cells in vivo, whereas carboxyl-truncated forms of SHIP are generated by proteolytic cleavage in vitro

Functional Characterization of the SHIP1-Domains Regarding Their Contribution to Inositol 5-Phosphatase Activity

The Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is a multidomain protein consisting of two protein-protein interaction domains, the Src homology 2 (SH2) domain, and the proline-rich region (PRR), as well as three phosphoinositide-binding domains, the pleckstrin homology-like (PHL) domain, the 5-phosphatase (5PPase) domain, and the C2 domain. SHIP1 is commonly known for its involvement in the regulation of the PI3K/AKT signaling pathway by dephosphorylation of phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) at the D5 position of the inositol ring. However, the functional role of each domain of SHIP1 for the regulation of its enzymatic activity is not well understood. To determine the contribution of the individual domains to catalytic activity, the full-length protein was compared with truncated constructs lacking one or more domain(s), regarding the substrate turnover (kcat) and catalytic efficiency (kcat/Km) towards ci8-PtdIns(3,4,5)P3. With this approach, it was possible to verify the allosteric activation of SHIP1 mediated by the C2 domain as described previously, while the PHL domain seemed instead to have a negative effect regarding catalytic efficiency. The full-length SHIP1 clearly displayed the highest turnover and the second-highest catalytic efficiency, showing the role of the SH2 domain and PRR not only in protein-protein interactions but also in catalysis. The SH2 domain increased substrate turnover but negatively affected catalytic efficiency. The linker between the SH2 and the PHL domains decreased the turnover number but positively influenced the catalytic efficiency. The PRR increased both the substrate turnover and the protein's catalytic efficiency. The regression analysis of the Michaelis-Menten graph revealed SHIP1 to be an allosteric enzyme, with the PRR and the linker being the most involved domains in that regard. In summary, our data indicate a complex regulation of the enzymatic activity of SHIP1 by its individual domains. While the C2 domain and PRR at the carboxy-terminus have a positive effect on enzymatic activity, the SH2 and PHL domain at the amino-terminus inhibit catalytic efficiency.

Spectral and Kinetic Study of 3-Methylquinoxalin-2-ones Photoreduced by Amino Acids: N-Phenylglycine Radical Chain Reactions and N-Acetyltryptophan Decarboxylation

Transient intermediates were identified in the photoreduction of 3-methylquinoxalin-2-one derivatives by N-phenylglycine, NPG, and N-acetyltryptophan, NAT. For both reductants it can be postulated a sequence of reaction comprising first a photoinduced single electron transfer followed by a proton transfer from the radical cation of the electron donor to the radical anion of the 3-methylquinoxalin-2-one giving rise to the reported products. The effect of the concentrations of NPG and the quinoxalin-2-one on the rate of photoconsumption of this last were quantified, and the lifetimes of the possible intermediates estimated. In the photoreduction by NAT, processes leading to the decarboxylation of NAT and radical adduct product compete with the expected SET from the indoyl N to the excited triplet of quinoxalin-2-ones as revealed by the detection of the deprotonated N-acetyltryptophan radical [NAT-H](•). This radical is formed almost instantly after the laser pulse and has a secondary delayed growth via a delayed proton transfer from the indoyl radical cation NAT(•+) to the quinoxalin-2-one radical anions. The decarboxylation of NAT that mimics C-terminus tryptophan in proteins is biologically relevant because might cause damages at cellular and the whole organism level. As far as we know this is the first report of a radical decarboxylation of N-acetyltryptophan leading to photoproducts.

PSTPIP2, a Protein Associated with Autoinflammatory Disease, Interacts with Inhibitory Enzymes SHIP1 and Csk

Mutations in the adaptor protein PSTPIP2 are the cause of the autoinflammatory disease chronic multifocal osteomyelitis in mice. This disease closely resembles the human disorder chronic recurrent multifocal osteomyelitis, characterized by sterile inflammation of the bones and often associated with inflammation in other organs, such as the skin. The most critical process in the disease's development is the enhanced production of IL-1β. This excessive IL-1β is likely produced by neutrophils. In addition, the increased activity of macrophages, osteoclasts, and megakaryocytes has also been described. However, the molecular mechanism of how PSTPIP2 deficiency results in this phenotype is poorly understood. Part of the PSTPIP2 inhibitory function is mediated by protein tyrosine phosphatases from the proline-, glutamic acid-, serine- and threonine-rich (PEST) family, which are known to interact with the central part of this protein, but other regions of PSTPIP2 not required for PEST-family phosphatase binding were also shown to be indispensable for PSTPIP2 function. In this article, we show that PSTPIP2 binds the inhibitory enzymes Csk and SHIP1. The interaction with SHIP1 is of particular importance because it binds to the critical tyrosine residues at the C terminus of PSTPIP2, which is known to be crucial for its PEST-phosphatase-independent inhibitory effects in different cellular systems. We demonstrate that in neutrophils this region is important for the PSTPIP2-mediated suppression of IL-1β processing and that SHIP1 inhibition results in the enhancement of this processing. We also describe deregulated neutrophil response to multiple activators, including silica, Ab aggregates, and LPS, which is suggestive of a rather generalized hypersensitivity of these cells to various external stimulants.

Enzymatic and non-enzymatic activities of SHIP-1 in signal transduction and cancer

PI3K cascade is a central signaling pathway regulating cell proliferation, growth, differentiation, and survival. Tight regulation of the PI3K signaling pathway is necessary to avoid aberrant cell proliferation and cancer development. Together with SHIP-1, the inositol phosphatases PTEN and SHIP-2 are the gatekeepers of this pathway. In this review, we will focus on SHIP-1 functions. Negative regulation of immune cell activation by SHIP-1 is well characterized. Besides its catalytic activity, SHIP-1 also displays non-enzymatic activity playing role in several immune pathways. Indeed, SHIP-1 exhibits several domains that mediate protein-protein interaction. This review emphasizes the negative regulation of immune cell activation by SHIP-1 that is mediated by its protein-protein interaction.

Posttranscriptional regulation of the p85α adapter subunit of phosphatidylinositol 3-kinase in human leukemia cells

Constitutive activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling has been observed in up to 70% of acute myeloid leukemia. Class I(A) PI3K consists of a catalytic subunit (p110α, p110β, p110δ) and an adapter subunit (p85α, p55α, p50α, p85β, p55γ). The p85α adapter subunit stabilizes the catalytic p110 subunit and recruits p110 to the plasma membrane. In addition, p85α inhibits the basal activity of p110α and can negatively regulate signal transduction, as shown for insulin and GM-CSF receptor signaling. Here, we describe that the expression of p85α is posttranscriptionally regulated in several human and murine leukemia cell lines and in a Hodgkin lymphoma cell line (CO) by translational repression. A detailed analysis of CO cells revealed that both wild type and a mutated p85α mRNA are detectable at similar ratios in the nucleus and polysomes. However, while the mutated p85α protein is expressed in CO cells, translation of the wild type p85α mRNA is completely inhibited. Ectopic expression of wild type p85α from a retroviral vector is suppressed in CO cells and in five out of six leukemia cell lines. Our data indicate that leukemia cells can regulate the expression of p85α by posttranscriptional regulation.

Role of SHIP in cancer

The SH2-containing inositol-5'-phosphatase, SHIP (or SHIP1), is a hematopoietic-restricted phosphatidylinositide phosphatase that translocates to the plasma membrane after extracellular stimulation and hydrolyzes the phosphatidylinositol-3-kinase-generated second messenger PI-3,4,5-P(3) to PI-3,4-P(2). As a result, SHIP dampens down PI-3,4,5-P(3)-mediated signaling and represses the proliferation, differentiation, survival, activation, and migration of hematopoietic cells. There are multiple lines of evidence suggesting that SHIP may act as a tumor suppressor during leukemogenesis and lymphomagenesis. Because of its ability to skew macrophage progenitors toward M1 macrophages and naïve T cells toward T helper 1 and T helper 17 cells, SHIP may play a critical role in activating the immune system to eradicate solid tumors. In this review, we will discuss the role of SHIP in hematopoietic cells and its therapeutic potential in terms of suppressing leukemias and lymphomas and manipulating the immune system to combat cancer.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

Regulation of the Src homology 2 domain-containing inositol 5'-phosphatase (SHIP1) by the cyclic AMP-dependent protein kinase

Many agents that activate hematopoietic cells use phos pha tidyl ino si tol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) to initiate signaling cascades. The SH2 domain-containing inositol 5' phosphatase, SHIP1, regulates hematopoietic cell function by opposing the action of phos pha tidyl ino si tol 3-kinase and reducing the levels of PtdIns 3,4,5-P(3). Activation of the cyclic AMP-de pend ent protein kinase (PKA) also opposes many of the pro-inflammatory responses of hematopoietic cells. We tested to see whether the activity of SHIP1 was regulated via phos pho ryl a tion with PKA. We prepared pure recombinant SHIP1 from HEK-293 cells and found it can be rapidly phos pho ryl a ted by PKA to a stoichiometry of 0.6 mol of PO(4)/mol of SHIP1. In (32)P-labeled HEK-293 cells transfected with SHIP1, stimulation with Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Sp-cAMPS) or activation of the beta-adrenergic receptor increased the phos pho ryl a tion state of SHIP1. Inhibition of protein phosphatase activity with okadaic acid also increased the phos pho ryl a tion of SHIP1. Phosphorylation of SHIP1 in vitro or in cells by PKA increased the 5' phosphatase activity of SHIP1 by 2-3-fold. Elevation of Ca(2+) in DT40 cells in response to B cell receptor cross-linking, an indicator of PtdIns 3,4,5-P(3) levels, was markedly blunted by pretreatment with Sp-cAMPS. This effect was absent in SHIP(-/-) DT40 cells showing that the effect of Sp-cAMPS in DT40 cells is SHIP1-de pend ent. Sp-cAMPS also blunted the ability of the B cell receptor to increase the phos pho ryl a tion of Akt in DT40 and A20 cells. Overall, activation of G protein-coupled receptors that raise cyclic AMP cause SHIP1 to be phosphorylated and stimulate its inositol phosphatase activity. These results outline a novel mechanism of SHIP1 regulation.

Reduced proliferation of CD34(+) cells from patients with acute myeloid leukemia after gene transfer of INPP5D

Acute myeloid leukemia (AML) is a malignant disease characterized by deregulated proliferation of immature myeloid cells. Constitutive activation of the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway is frequently detected in approximately 50-70% of AML patients. The gene INPP5D encodes the SH2-containing inositol 5-phosphatase 1 (SHIP1), which is a negative regulator of PI3K/AKT signaling. After lentiviral-mediated gene transfer of INPP5D into CD34(+) cells derived from AML patients (n=12) the granulocyte macrophage-colony stimulating factor (GM-CSF)-dependent proliferation was reduced in all samples analyzed (average 86%; range 72-93%). An enzymatically inactive form of SHIP1 (D672A) had no effect. In addition, SHIP1 reduced the autonomous proliferation of CD34(+) cells from a patient with a secondary AML who had a very high peripheral blast count (300 x 10(9) l(-1)). These data show that SHIP1 can effectively block GM-CSF-dependent and autonomous proliferation of AML cells.

Restoration of SHIP activity in a human leukemia cell line downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle

The inositol 5-phosphatase SHIP (SHIP-1) is a negative regulator of signal transduction in hematopoietic cells and targeted disruption of SHIP in mice leads to a myeloproliferative disorder. We analyzed the effects of SHIP on the human leukemia cell line Jurkat in which expression of endogenous SHIP protein is not detectable. Restoration of SHIP expression in Jurkat cells with an inducible expression system caused a 69% reduction of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and a 65% reduction of Akt kinase activity, which was associated with reduced phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) (Ser-9) without changing the phosphorylation of Bad (Ser-136), FKHR (Ser-256) or MAPK (Thr-202/Tyr-204). SHIP-expressing Jurkat cells showed an increased transit time through the G1 phase of the cell cycle, but SHIP did not cause a complete cell cycle arrest or apoptosis. Extension of the G1 phase was associated with an increased stability of the cell cycle inhibitor p27(Kip1) and reduced phosphorylation of the retinoblastoma protein Rb at serine residue 780. Our data indicate that restoration of SHIP activity in a human leukemia cell line, which has lost expression of endogenous SHIP, downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3beta signaling and leads to an increased transit time through the G1 phase of the cell cycle.

The role of SHIP in cytokine-induced signaling

The phosphatidylinositol (PI)-3 kinase (PI3K) pathway plays a central role in regulating many biological processes via the generation of the key second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P3). This membrane-associated phospholipid, which is rapidly, albeit transiently, synthesized from PI-4,5-P2 by PI3K in response to a diverse array of extracellular stimuli, attracts pleckstrin homology (PH) domain-containing proteins to membranes to mediate its many effects. To ensure that the activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed tumor suppressor PTEN hydrolyzes PI-3,4,5-P3 back to PI-4,5-P2 while the 145-kDa hemopoietic-restricted SH2-containing inositol 5'- phosphatase, SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP (sSHIP) and the more widely expressed 150-kDa SHIP2 hydrolyze PI-3,4,5-P3 to PI-3,4-P2. In this review we will concentrate on the properties of the three SHIPs, with special emphasis being placed on the role that SHIP plays in cytokine-induced signaling.

SHIP, SHIP2, and PTEN activities are regulated in vivo by modulation of their protein levels: SHIP is up-regulated in macrophages and mast cells by lipopolysaccharide

The phosphatidylinositol-3 kinase (PI3K) pathway plays a central role in regulating numerous biologic processes, including survival, adhesion, migration, metabolic activity, proliferation, differentiation, and end cell activation through the generation of the potent second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P(3)). To ensure that activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed 54-kDa tumor suppressor PTEN hydrolyzes PI-3,4,5-P(3) to PI-4,5-P(2), whereas the 145-kDa hematopoietic-restricted SH2-containing inositol 5'-phosphatase SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP sSHIP, and the more widely expressed 150-kDa SHIP2 break it down to PI-3,4-P(2). In this review, we focus on the properties of these phospholipid phosphatases and summarize recent data showing that the activities of these negative regulators often are modulated by simply altering their protein levels. We also highlight the critical role that SHIP plays in lipopolysaccharide-induced macrophage activation and in endotoxin tolerance.

SH2-containing inositol 5-phosphatases 1 and 2 in blood platelets: their interactions and roles in the control of phosphatidylinositol 3,4,5-trisphosphate levels

Src homology domain 2-containing inositol 5-phosphatases 1 and 2 (SHIP1 and SHIP2) are capable of dephosphorylating the second messenger PtdIns(3,4,5) P3 (phosphatidylinositol 3,4,5-trisphosphate) and interacting with several signalling proteins. SHIP1 is essentially expressed in haematopoietic cells, whereas SHIP2, a closely related enzyme, is ubiquitous. In the present study, we show that SHIP1 and SHIP2 are expressed as functional PtdIns(3,4,5) P3 5-phosphatases in human blood platelets and are capable of interacting when these two lipid phosphatases are co-expressed, either naturally (platelets and A20 B lymphoma cells) or artificially (COS-7 cells). Using COS-7 cells transfected with deletion mutants of SHIP2, we demonstrate that the Src homology domain 2 of SHIP2 is the minimal and sufficient protein motif responsible for the interaction between the two phosphatases. These results prompted us to investigate the relative importance of SHIP1 and SHIP2 in the control of PtdIns(3,4,5) P3 levels in platelets using homozygous or heterozygous SHIP1- or SHIP2-deficient mice. Our results strongly suggest that SHIP1, rather than SHIP2, plays a major role in controlling PtdIns(3,4,5) P3 levels in response to thrombin or collagen activation of mouse blood platelets.

Dual-specificity phosphatase Pyst2-L is constitutively highly expressed in myeloid leukemia and other malignant cells

Northern blotting confirmed previous results indicating that the mitogen-activated protein kinase (MAPK) phosphatase Pyst2-L was highly expressed in leukocytes obtained from acute myeloid leukemia (AML) patients. High levels of Pyst2-L mRNA were expressed in bone marrow (BM) and peripheral leukocytes from nine AML and acute lymphoblastic leukemia (ALL) patients. BM from healthy individuals expressed very low levels of Pyst2-L. Whereas high levels of Pyst2-L mRNA and protein were detected in several leukemia cell lines, Pyst2-L mRNA was detected neither in 33/34 samples of normal peripheral blood mononuclear cells (PBMC) nor in leukocyte fractions enriched with CD34+ cells. Certain solid tumor and lymphoblastoid cell lines expressed high levels of Pyst2-L mRNA. In view of the association of Pyst2-L to MAPK signaling cascades, we tested if cell activation, a process involving MAPK signaling, influences Pyst2-L expression. Indeed, activation of T cells and endothelial cells increased Pyst2-L in these cells. Furthermore, TPA, a known MAPK activator, induces the expression of both Pyst2-L mRNA as well as the Pyst2-L protein in leukemia cells. This induction was partially inhibited by PD098059, an Mek1/2-specific inhibitor. Based on the results of this and previous studies, we hypothesize that the high levels of Pyst2-L detected in the active state of AML and ALL diseases and in other types of cancer reflect an altered MAPK signaling pathway in such malignant processes. This alteration may be the result of a failed attempt to counter the constitutive activation of MAPK in transformed cells or alternatively, may represent the activated state of such cells.

Evidence for a positive role of SHIP in the BCR-ABL-mediated transformation of primitive murine hematopoietic cells and in human chronic myeloid leukemia

Previous studies suggested that the SH2-containing inositol-5-phosphatase (SHIP) may play a tumor suppressor-like function in BCR-ABL-mediated leukemogenesis. To investigate this possibility, we first developed a new assay for quantitating transplantable multilineage leukemia-initiating cells (L-ICs) in hematopoietic stem cell (HSC)-enriched mouse bone marrow (BM) cells transduced with a BCR-ABL-GFP (green fluorescent protein) retrovirus. The frequency of L-ICs (1 of 430 Sca-1+lin- cells) was 7-fold lower than the frequency of HSCs in the Sca-1+lin- subset transduced with a control virus (1 of 65 cells). Forced BCRABL expression was also accompanied by a loss of regular HSC activity consistent with the acquisition of an increased probability of differentiation. Interestingly, the frequency and in vivo behavior of wild-type (+/+) and SHIP-/- L-ICs were indistinguishable, and in vitro, Sca-1+lin- BCR-ABL-transduced SHIP-/- cells showed a modestly reduced factor independence. Comparison of different populations of cells from patients with chronic myeloid leukemia (CML) in chronic phase and normal human BM showed that the reduced expression of full-length SHIP proteins seen in the more mature (CD34-lin+) leukemic cells is not mirrored in the more primitive (CD34+lin-) leukemic cells. Thus, SHIP expression appears to be differently altered in the early and late stages of differentiation of BCR-ABL-transformed cells, underscoring the importance of the cellular context in which its mechanistic effects are analyzed.

Survival of acute myeloid leukemia cells requires PI3 kinase activation

The mechanisms that regulate the growth and survival of acute myeloid leukemia (AML) cells are largely unknown. We hypothesized that constitutive activation of phosphatidyl-inositide 3 kinase (PI3 kinase) could regulate survival in primary cells from patients with AML. Here we demonstrate that Akt, a critical substrate of PI3 kinase, is activated in AML blasts. In a short-term culture system, most AML patient samples showed a dose-dependent decrease in survival after incubation with the PI3 kinase inhibitor LY294002. This decrease in survival was partially due to the induction of apoptosis. Furthermore, we have shown that p70 S6 kinase and 4EBP-1, downstream mediators of Akt signaling, also are phosphorylated in AML blasts. Phosphorylation of these proteins is inhibited by the mTOR inhibitor RAD001. Incubation of AML blasts with RAD001 induces only a small decrease in survival of the cells; however, when combined with Ara-C, RAD001 enhances the toxicity of Ara-C. These results demonstrate that constitutive activation of the PI3 kinase pathway is necessary for the survival of AML blasts and that targeting of this pathway with pharmacologic inhibitors may be of clinical benefit in treatment of AML.

Activin/TGF-beta induce apoptosis through Smad-dependent expression of the lipid phosphatase SHIP

Members of the transforming growth factor beta (TGF-beta) family regulate fundamental physiological processes, such as cell growth, differentiation and apoptosis, in almost all cell types. As a result, defects in TGF-beta signalling pathways have been linked to uncontrolled cellular proliferation and carcinogenesis. Here, we explored the signal transduction mechanisms downstream of the activin/TGF-beta receptors that result in cell growth arrest and apoptosis. We show that in haematopoietic cells, TGF-beta family members regulate apoptosis through expression of the inositol phosphatase SHIP (Src homology 2 (SH2) domain-containing 5' inositol phosphatase), a central regulator of phospholipid metabolism. We also demonstrated that the Smad pathway is required in the transcriptional regulation of the SHIP gene. Activin/TGF-beta-induced expression of SHIP results in intracellular changes in the pool of phospholipids, as well as in inhibition of both Akt/PKB (protein kinase B) phosphorylation and cell survival. Our results link phospholipid metabolism to activin/TGF-beta-mediated apoptosis and define TGF-beta family members as potent inducers of SHIP expression.

Proteolytic degradation of Smad4 in extracts of AML blasts

Loss of transforming growth factor (TGF) beta signaling has been implicated in malignant transformation of various tissues. To investigate a potential role of Smad4 in acute myeloid leukemia (AML), the expression of Smad4 was determined in blast cells from AML patients. Western analysis of nuclear extracts of nine AML samples indicated the absence of Smad4 protein in two cases. Smad4 RT-PCR analysis of these cases indicated normal Smad4 mRNA expression, and sequencing of one of these cases revealed no mutations as compared to wild type Smad4. Next, it was investigated whether Smad4 protein from these AML cases was subject to proteolytic degradation by incubating cell extracts of these Smad4-negative AML cells with extracts from COS-7 cells in which a tagged Smad4 was overexpressed. Inhibitor studies indicated that the extracts of AML blasts lacking Smad4 possessed a serine-dependent proteolytic activity, capable of degrading Smad4. Transfection studies using an SBE containing reporter construct as well as RT-PCR analysis of endogenous TGFbeta1 responsive genes indicated that the AML blasts were still able to respond to TGFbeta1, despite the observed degradation of Smad4. It was, therefore, concluded that the degradation of Smad4 was possibly AML subtype-dependent, in vitro phenomenon, occurring during the preparation of nuclear and cellular extracts despite the addition of a protease inhibitor cocktail. The results indicate that care should be taken when interpreting data obtained from protein expression studies using AML blast cells.

Expression of a mutated form of the p85alpha regulatory subunit of phosphatidylinositol 3-kinase in a Hodgkin's lymphoma-derived cell line (CO)

Phosphatidylinositol (PI) 3-kinase plays an important role in a variety of biological processes, including proliferation and apoptosis. PI3-kinase is a heterodimer consisting of an 85 kDa adapter protein (p85) containing one SH3 domain and two SH2 domains and a 110 kDa catalytic subunit (p110). Recently an oncogenic form of p85 named p65-PI3K lacking the C-terminal SH2 domain has been cloned from an irradiation-induced murine thymic lymphoma and transgenic mice expressing p65-PI3K in T lymphocytes develop a lymphoproliferative disorder. Here we describe the cloning of a C-terminal truncated form of p85 expressed in a human lymphoma cell line (CO) with a T cell phenotype derived from a patient with Hodgkin's disease. As a result of a frame-shift mutation at amino acid 636, p76 is lacking most of the C-terminal SH2 domain, but contains the inter-SH2 domain and is associated with an active form of PI3-kinase. A PI3-kinase-dependent constitutive activation of Akt was detected in CO cells which was only partially reduced after serum starvation. Treatment of CO cells with the PI3-kinase inhibitor wortmannin resulted in a concentration-dependent inhibition of cell proliferation associated with an increased number of apoptotic cells. This is the first detection of a mutated form of the p85 subunit of PI3-kinase in human hematopoietic cells further underlining a potential role of PI3-kinase/Akt signaling in human leukemogenesis.